NO. 3 EMBRYOLOGY OF FLEAS KESSEL 45 



The anterior and posterior extremities, therefore, become so flexed 

 that a cross-section of the egg may cut four complete sections through 

 the embryo. The embryo retains its curled position until the time of 

 hatching, after the sixth day of development. 



DIGESTIVE TRACT 



The fore-intestine, comprising the oesophagus and proventriculus 

 in the late embryo, has its origin, in fleas as in other insects, in an 

 ectodermal invagination known as the stomodaeum. This invagination 

 arises directly over the position of the anterior mesenteron rudiment, 

 becoming evident soon after the ectoderm has closed over the sunken 

 entoderm. The stomodaeum deepens very gradually, keeping pace 

 with the shrinkage of the yolk. As it deepens it pushes the entoderm 

 of the anterior mesenteron rudiment before it. It ultimately extends 

 posteriorly about one fourth of the length of the egg (pi. 3, fig. 34; 

 pi. 10, fig. 75 ; pi. 11, fig. 82). 



The hind-intestine arises in a somewhat similar manner although 

 there are several differences to be noted. Like the fore-intestine, it 

 is derived from an ectodermal invagination which in this instance is 

 termed the proctodaeum (pi. 3, figs. 35, 36; pi. 10, fig. 75). It may 

 be said that the relationship between the proctodaeum and posterior 

 mesenteron rudiment is fundamentally the same as that which exists 

 between the stomodaeum and anterior entodermal rudiment. In the 

 case of the proctodaeum and posterior mesenteron rudiment, however, 

 the relationship is made more obscure in the earlier stages of develop- 

 ment by the involution of the posterior region of the germ band. The 

 point at which the embryonic band sinks inward during this process 

 is almost immediately over the position of the posterior mesenteron 

 rudiment and therefore somewhat anterior to the posterior extremity 

 of the germ band. This is also the point at which the proctodaeum 

 pushed inward so that the first part of the resulting invagination to 

 form appears to belong to both the proctodaeum and the amniotic 

 cavity and has consequently been termed the amnio-proctodaeal cavity. 

 However, since this first portion of the cavity of the invagination is 

 bounded on both sides by the embryonic rudiment, it is definitely the 

 lumen of the proctodaeum. Only later, as the involution of the 

 posterior part of the germ band becomes more pronounced, is a 

 portion of the extra-embryonic blastoderm drawn into the vitellus to 

 form the internal (with reference to the yolk) section of the amnion 

 (pi. 7, figs. 56, 59). That part of the cavity which is bounded on one 

 side by the amnion is the only portion of the entire invagination which 



